The agency didn't spell out which countries should start screening passengers, but noted that the Ebola outbreak involves transmission in Guinea, Liberia and Sierra Leona and a "small number of people in Nigeria."

All countries, even those unaffected by the outbreak in West Africa, need to strengthen their ability to detect and immediately contain new cases without doing anything that unnecessarily interferes with international travel or trade, the agency said. But countries don't need to impose travel restrictions and active screening of passengers if they do not share borders with Ebola-affected countries, it said.

Authorities in Liberia, Sierra Leone and Guinea say they are already closely inspecting departing passengers for signs of fever or illness.

The risk of the Ebola virus being transmitted during air travel is low because unlike infections such as influenza or tuberculosis, it is not spread by breathing air and airborne particles from an infected person.

Nonetheless, the World Health Organization said anyone with an illness consistent with the virus should not be allowed to travel normally and all passengers should routinely wash their hands and avoid direct contact with body fluids of infected people.

"Transmission requires direct contact with blood, secretions, organs or other body fluids of infected living or dead persons or animals, all unlikely exposures for the average traveller," the agency said in a statement.

The only way to contain the disease, for which there is no licensed treatment, is by isolating the sick and closely watching for signs of infection in those they have come into contact with. A person usually has no symptoms for two to 21 days, but after the incubation period the symptoms include fever, weakness, muscle pain, headache and sore throat, according to WHO. Then comes vomiting, diarrhea, rash, and in some cases, bleeding.

The Geneva-based agency has been criticized by non-U.N. health organizations as being slow to call for an emergency response to the Ebola crisis.

Some countries have banned direct flights to states hit by the disease. British Airways, Kenya Airways and a number of regional carriers have also cancelled flights to the capitals of Sierra Leone and Liberia despite the WHO's recommendation that no travel or trade bans be put in place.

Last month, a Liberian-American man infected with Ebola boarded a flight from Liberia to Nigeria and died days later; 11 people who came into contact with him have been infected.

A task force to monitor the impact of the Ebola virus on travel and transport has been launched by the U.N. health agency along with the International Civil Aviation Organization, the World Tourism Organization, the Airports Council International, the International Air Transport Association and the World Travel and Tourism Council.

Most of the infections in Liberia, Guinea and Sierra Leone have occurred when family members or friends act as caregivers for those who are ill or during burials that don't follow strict infection prevention and control measures, according to WHO officials.